Spatial rainfall variability and runoff response during an extreme event in a semi-arid catchment in the South Pare Mountains, Tanzania
Abstract. This paper describes an extreme flood event that occurred in the South Pare Mountains in northern Tanzania. A high spatial and temporal resolution data set has been gathered in a previously ungauged catchment. This data was analysed using a multi-method approach, to gather information about the processes that generated the flood event. On 1 March 2006, extreme rainfall occurred in the Makanya catchment, (300 km2), where up to 100 mm were recorded in Bangalala village in only 3 h. The flood was devastating, inundating large parts of the flood plain. The spatial variability of the rainfall during the event was very large, even in areas with the same altitude. The Vudee sub-catchment (25.8 km2) was in the centre of the rainfall event, receiving about 75 mm in 3 h divided over the two upstream tributaries: the Upper-Vudee and Ndolwa. The peak flow at the weir site has been determined using the slope-area method and gradually varied flow calculations, indicating a peak discharge of 32 m3 s−1. Rise and fall of the flood was very sharp, with the peak flow occurring just one hour after the peak of the rainfall. The flow receded to 1% of the maximum flow within 24 h. Hydrograph separation using hydrochemical parameters indicates that at the floodpeak 50% of the flow was generated by direct surface runoff (also indicated by the large amount of sediments in the samples), whereas the recession originated from displaced groundwater (>90%). The subsequent base flow in the river remained at 75 l s−1 for the rest of the season, which is substantially higher than the normal base flow observed during the previous rainy seasons (15 l s−1) indicating significant groundwater recharge during this extreme event.